Client-side execution via WebAssembly is the right call — no code leaving the browser removes the trust barrier that kills most online editors for anything beyond toy snippets. The multi-language range (C#, Rust, Python, SQL with both SQLite and DuckDB) is unusually broad for a browser-based tool.
One feature that would make it genuinely useful beyond prototyping: shareable permalink to a code state. Half the value of an in-browser runner is pasting "here's the exact thing, click run" into a discussion thread or docs. If that's already there, even better.
DuckDB support is a nice touch — that's the one most online SQL editors skip. Will try it out.